Alexander Balandin (left) and Guanxiong Liu (right) fabricated the VCO in a cleanroom at the UCR's Center for Nanoscale Science and Engineering. Photo: UC Riverside.
Alexander Balandin (left) and Guanxiong Liu (right) fabricated the VCO in a cleanroom at the UCR's Center for Nanoscale Science and Engineering. Photo: UC Riverside.

Graphene may have emerged as a highly promising two-dimensional (2D) crystal, but the future of electronics could include two other nanomaterials as well, according to a new study by researchers at the University of California, Riverside (UCR) and the University of Georgia.

In a paper published in Nature Nanotechnology, the researchers report the integration of three very different 2D materials to yield a simple, compact and high-speed voltage-controlled oscillator (VCO). A VCO is an electronic oscillator whose oscillation frequency is controlled by the voltage input.

The VCO is the first useful device to exploit the potential of using charge-density waves to modulate an electrical current through a 2D material. This new technology could offer an ultralow-power alternative to conventional silicon-based VCOs, which are used in thousands of applications from computers to clocks to radios. The thin, flexible nature of the device would also make it ideal for use in wearable technologies.

Graphene is a single layer of carbon atoms that exhibits exceptional electrical and thermal conductivities, and shows considerable promise as a successor to silicon-based transistors. However, its application has been limited by its inability to function as a semiconductor, which is critical for the 'on-off' switching operations performed by electronic components.

To overcome this shortfall, the researchers turned to another 2D nanomaterial, tantalum sulfide (TaS2). They showed that voltage-induced changes in the atomic structure of a polytype of TaS2 (in which layers of TaS2 are stacked in a specific sequence) allowed it to function as an electrical switch at room temperature – a requirement for practical applications.

"There are many charge-density wave materials that have interesting electrical switching properties; however, most of them reveal these properties at very low temperature only," said Alexander Balandin, professor of electrical and computer engineering in UCR's Bourns College of Engineering, who led the research team. "The particular polytype of TaS2 that we used can have abrupt changes in resistance above room temperature. That made a crucial difference."

To protect the TaS2 from environmental damage, the researchers coated it with another 2D material, hexagonal boron nitrate, which can prevent oxidation. By pairing the boron nitride-capped TaS2 with graphene, the team constructed a three-layer VCO that could pave the way for other post-silicon electronics. In the proposed design, graphene functions as an integrated tunable load resistor for allowing precise voltage control of the current and VCO frequency. This prototype UCR device operates at the megahertz (MHz) frequencies used in radios; however, the extremely fast physical processes that define the device functionality should allow for the operation frequency to be increased all the way to terahertz frequencies.

Balandin said this integrated system is the first example of a functional VCO made from 2D materials that operates at room temperature.

"It is difficult to compete with silicon, which has been used and improved for the past 50 years, " he said. "However, we believe our device shows a unique integration of three very different 2D materials, which utilizes the intrinsic properties of each of these materials. The device can potentially become a low-power alternative to conventional silicon technologies in many different applications."

Graphene’s function in the proposed 2D device overcomes the problem associated with its lack of an energy band gap, which has so far prevented graphene being used as the transistor channel material. The extremely high thermal conductivity of graphene comes as an added benefit in the device structure, by facilitating heat removal. The unique heat conduction properties of graphene were experimentally discovered and theoretically explained in 2008 by Balandin's group at UCR.

The Balandin group also demonstrated the first integrated graphene heat spreaders for high-power transistors and light-emitting diodes. "In those applications, graphene was used exclusively as heat conducting material. Its thermal conductivity was the main property. In the present device, we utilize both electrical and thermal conductivity of graphene," Balandin explained.

This story is adapted from material from the University of California, Riverside, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.